Intracochlear injection of the fluorescent retrograde neuronal tracers fast blue and diamidino yellow was used to investigate the distribution within the brainstem of neurones projecting to the cochlea in the guinea pig. The overall pattern of distribution of cells within the brainstem auditory nuclei was the same for both tracers and was also in broad agreement with recent studies in this species using horse-radish peroxidase as the neuronal tracer. However, the total number of neurones found (mean of 1234 projecting to each cochlea) was significantly greater than that reported using horseradish peroxidase, largely as a result of more small labelled neurones being detected within the lateral superior olivary nucleus ipsilateral to the injected cochlea. The yield of labelled cells was greatest in animals in which care was taken to perfuse the whole length of the cochlear epithelium. After bilateral injections of both tracers, double-labelled cells were found in small numbers within all the large neurone medial system nuclei and the ventral nucleus of the lateral lemniscus. It was concluded that between 1-5% of the medial system neurones project to both cochleas.